1. Field of the Invention
The present invention relates to an ophthalmic measurement apparatus, and more particularly to an ophthalmic measurement apparatus for observing or measuring an eye to be examined by positioning the apparatus so as to have a predetermined positional relationship with the eye.
2. Description of Related Art
Regarding an ophthalmic measurement apparatus which requires positional adjustment so as to have a predetermined positional relationship between an eye to be examined and an apparatus, it is conventionally known to utilize a corneal reflection point. In such a method, the apparatus projects an alignment target onto a cornea of the eye, and photographs a corneal reflection point with photographing elements included in an alignment optical system so as to display the image of the corneal reflection point on a monitor. Alignment is to be performed with observing the image. To have a predetermined positional relationship between the image of a corneal reflection point and a reticle mark (an alignment mark), which is generated either optically or electrically, positional adjustment is made in horizontal directions and vertical directions. In order to make the image of a corneal reflection point the smallest, adjustment in forward and backward directions is made by judging from a blur of the image.
Referring to a measurement means for measuring an eye to be examined, it is known, for example, an apparatus to detect a placido ring target which is projected onto relatively broad part of a cornea of the eye with photographing elements included in a photographing optical system, and measures distribution of corneal curvature in detail based on the detected signal. In this case, a focal depth is made to be comparatively deep in a manner of providing a diaphragm which is adjusted to be appropriate in size in an optical path of a photographing optical system, so that a sharp photographed image over an anterior part of the eye from its center to its periphery can be obtained without being influenced by a position of a projected target image on a cornea of the eye.
However, in a photographing optical system which is required to photograph an image sharply as described above, when an alignment utilizing blur of an image of a corneal reflection point is to be performed, different sets of photographing elements are required for a photographing optical system and for an alignment optical system due to the difference in a focal depth which each optical system requires.